Titus Livius known in English as Livy (1st C BC) and the
Greek writer Polibius are perhaps the best literary source regarding the
Roman army, wars and battles of the earliest history of Rome: They were
more like cattle raids and skirmishes against the tribes and settlements
in the vicinity of the Tiber. There would have been some pitched battles
although they would be more the exception than the general rule. The
armies and troops involved were closer to bands of warriors than the
Roman army we have in our collective imagination. Such groups would
include a leader – a member of the local nobility ("Patricians")
– together with supporters, family members and others belonging to his
family group ("gentes"). We have a description of this in Livy Bk2, 49.

A step closer to the developed Roman army we know was achieved around
the 6th century BC, during the reign and reforms of
King Servius
Tullius.

Servius’ reforms
included military reform and the census: the latter was clearly
necessary to enable management of public affairs such as voting and
taxes but also to facilitate the creation of a military force built
around the city’s citizens because the census enabled citizens to be
subdivided into classes of wealth.

This link between voting rights, wealth (land ownership) and military
duty was highly significant in that it constituted the basis the Roman
army and Roman citizenship: those with a right to vote and wage war were
those who had property to defend and an interest in disseminating Roman
law.

The old system based on farmer-soldiers was eventually doomed to fail
but the ancient Romans were quick to adapt their approach. Factors
forcing change were:

Farming requires its own particular seasons of work in order to
produce food and income: wars on the other hand are not intrinsically
seasonal. The "war season" was an early concept which eventually fell
out of use: Imagine the strategic advantage of being able to wage war
when your enemy is forced to tend to the land lest he be left with no
winter food…..

Wars were increasingly long in duration and far from home.

Territorial expansion, increasingly extended borders and
increasingly tough enemies in extended wars such as the Samnite and
Punic wars implied an increasing need of men to fill the file and ranks
of the Roman army.

Territorial expansion meant greater overall wealth for Rome but this
wealth was increasingly concentrated in the hands of a few, whilst the
broader population was relatively impoverished. This meant that the
number of people falling into the various census classes was changing
greatly with many actually falling off the bottom of the scale and
forming part of the increasingly large proletariat.

The relative lack of social and military organization during the 5th
century meant that many of the wars undertaken were inconclusive and
generally less fruitful in terms of booty and land with which to pay out
rewards. The difficult economic situation lead many to incur debts and
in need of a means to find income.

Result: The recruitment approach began to change around the 4th
century when Rome began to pay its soldiers so as to enable them to
serve a longer period of military service as well as progressively
lowering the wealth individuals were expected to have in order to form
part of the various classes of soldier. In 405BC the consuls Albinus and
Camillus, for the first time, conscripted 2000 men who lacked the wealth
requirements for service in the army.

In terms of the Roman army, the issue of war booty distribution (or
lack thereof) had various effects:

Lack of wealth distribution caused great social upheaval within Rome
and it is curious to note that the Gracchi brothers, upholders of the
plebeian rights to more equal land distribution had close family ties
with some of Rome’s greatest generals against Carthage. Their reforms
found great opposition from the nobility. From a practical army
enlisting point of view the heavy imbalance in wealth distribution
forced a shift in the census parameters by which individuals were
measured for enlisting in the army – this was brought to its natural
extreme with Marius’ army prototype in 107BC made of fully professional
soldiers driven more by the promise of personal gain in the booty than
by the traditional honour of representing their country.

Outside Rome, the imbalance engendered the Social Wars during the
years 91-88BC – many of Rome’s Italic allies rebelled against the unfair
distribution of wealth (they had been driven to poverty) and created an
alliance amongst themselves against Rome. Several years of war and
battle saw a slow but progressive Roman victory, but nevertheless the
indomitable Samnites held out and managed to force Roman law for greater
equality – winning Roman citizenship and voting rights for themselves
and the allies (allies = "Soci" hence "Social" wars). Ie significant
social changes ensued in Rome itself.

According to the writer Polibius we learn that in the early days all
citizens within the ages of 17 and 46 years of age owning over 11,000
aes were obliged by law to attend the annual military conscription
process known as "dilectus" on the Capitoline hill. When times became
harder or need for men grew then the minimum wealth hurdle was reduced.
The number of years of service for each man was limited to something in
the region of 16 as foot soldier and 10 as cavalry. As the years
progressed the conscription rules were changed and refined, but always
with the concept of maximizing the use of men in the army whilst also
placing an ultimate pensionable limit on their service. The longest
period of consecutive service was placed at 6 but this didn’t mean
prevent you from being called back to service for many more campaigns or
periods of annual service.

A definitive shift came about with Gaius Marius around 107BC; himself
a man of the people who had made it to the top of public ranks, strongly
supported into Consular office by the Plebeians against the Patrician
noble classes. Rome had seen a number of relatively ineffectual years of
campaigning in Africa and more importantly was threatened by Germanic
barbarians invading Italy from the north. Marius broke all conventions
by taking a personal unilateral initiative in conscripting and creating
an army made up purely of paid volunteers from the lower classes:
individuals who were driven almost entirely by personal interest in the
booty they would receive rather than by nationalistic sentiment. This
large mass of professional soldiers, motivated almost entirely by
personal gain was to change the future shape of ancient Roman politics
as they were the mainstay of any figure wishing to clinch power. It is
hardly surprising that the shift came about just when the civil wars
were about to begin.

Notwithstanding the change to a professional army, the tight link
between civilian life and military life remained intrinsic to the Roman
military system, especially in the early years of the Roman conquest of
Italy, but also through the republican and imperial periods: The highest
military ranks were held by high ranking magistrates such as Consuls and
Praetors who had been voted into office by the public at large. The link
between military and civilian spheres became even stronger when the
supreme commander of military power was the emperor himself and
vice-versa in the latter years of the empire the emperor was often a
person who held military command. Good examples of this are emperor
Vespasian (who turned out to be an altogether good emperor) and emperor
Constantine. There were of course bad examples also.

Difficult political situations were frequently caused by the
emperor’s or commanders need to reward his veteran soldiers by giving
them some land to farm and live on. An interesting example of this was
Augustus (then known as Octavian) who had the makings of a military
revolt on his hands in Italy whilst having to fight against Marc Anthony
and Cleopatra in the eastern Mediterranean. Clearly this land was
frequently procured by confiscating the land and belongings of those who
had been vanquished.

To the number of soldiers coming from Rome’s citizens we have to add
at least an equal number of legionaries provided by Rome’s allied or
vanquished populations. The general rule was therefore that the Roman
legion would be more than doubled in size by a supporting wing ("ala")
of allied militia. Allied cavalry support was about three times the
Roman cavalry – ie a legion, which generally included 300 Roman cavalry,
would be supported by a further 900 allied cavalry.

The forces and officers provided by the socii were commanded by a
number of Roman officers nominated by the consuls.

It is estimated that the Italic allies of Rome could provide
something in the region of 250,000-300,000 men including some 35,000
cavalry. At the time of the Punic wars against Carthage and general
Hannibal Rome came to mobilize as many as 25 legions: 120,000 men, which
when coupled with allied forces would have added to something akin to
250,000 men. Livy tells us that the devastating loss at the battle of
Cannae against Hannibal cost Rome a full 50,000 men , but clearly whilst
a large number, this was only a fraction of the total number of forces
available.

In order to have a clear understanding of how the Roman army changed
in shape and form, it is also necessary to appreciate how Rome’s wars
and expansion from Rome, across Italy, into Sicily and then across the
Mediterranean: The changing needs of terrain, distance, logistics and
duration, not to mention the changing types of adversary meant that the
Roman army had to adapt and change in form and organization.

Shift from the greek type compact Phalanx to the more widely spread
manipulus and legions. This was likely a gradual shift, but likely
around the time of the Samnite wars in the 4th century BC
(Sallust – Bellum Catilinae bk 51)

Around this period we also see the possible introduction of a
professional army model – a very basic pay enabled the soldiers to stay
away from their land and work and to be motivated by a cut of the booty.

Adoption of various types of armament such as the Hispanic gladius
as a sword and the ply-wood scutum rather than the hoplitic round shield

Building of a navy from scratch, adaptation of the enemy’s
(Carthaginian) ship type and in a short time actually beat the
Carthaginians with it (various battles around 260-241BC).

The Romans of the kingdom and early republic learned from the
neighbouring Etruscans how to use the phalanx, similar in style to that
of the Greeks who had their colonies to the south of Rome. The Roman
legion of this period was built of Roman citizens sufficiently rich to
pay their own weapons and armor: The census introduced by Servius
Tullius described above allowed the construction of the phalanx
according to citizens of given levels of wealth and corresponding
personal armament:

Heavily armed foot soldiers "Hastati" (spear men) and
"Principes" (leaders/chieftains). The Hastati were the younger,
Principes were middle aged.
These men carried:

the Italic oval "Scutum" or the Greek/Etruscan circular
shield "clypeus" some 90cm in diameter.

As of around 250BC they carried particular spears known
as "pilum", likely one heavy and the other lighter. These
spears were particular in that they had a very heavy round
or square section pyramid-shaped tip with a thin neck and
metal shaft measuring some 1.4m in length half of which was
inserted into a wooden shaft making a total of 2m.

The Pilum spear would be hurled when some 15-30m from the
enemy and had very great penetrating power, but at the very
least it would stick into enemy shields, bend at the neck
and render the shield unwieldy for further use.

It is likely that the concept was borrowed and adapted from
Iberian mercenary enemies during the first Punic war, just
like the gladius. Others suggest it was learned from the
Samnites, ie before the Punic wars and that originally the
pointed metal shaft was intended as something to be stuck in
the ground for defensive purposes, only later transformed
into something to be hurled at the enemy.

They used the famous "gladius" which they had learned
and adapted from Spanish mercenary enemies. The gladius
could be used both as a cutting and jabbing wheapon, likely
most effective with the jab.

"Triarii": Older but experienced soldiers. These were armed
similarly to the Hastati and Principes, they likely carried a
the oblong "scutum" and a long spear rather than the pilum,
likely a couple of meters in length (6ft).

The cavalry - "Equites" bringing up the sides. As well as
paying for their armour would have to pay and keep a horse.

"Velites" or "Leves" were lightly armed foot soldiers. They
had little if any training, no commanders of their own and light
armament, perhaps some javelins or a sling. They might wear a
"Galea" on their head: a leather helmet which gave them the
appearance of a wolf: As Virgil (Aen. vii, 688) tells us:

These different classes of soldier were laid out in lines, like a
wall, facing the enemy, starting with the heavily armed soldiers as in
the order described above. Each line broken up into 10 building blocks
or units called "manipulus" (Other accounts eg Polibius suggest
15 manipuli). The following row would be lined up behind the one in
front but with the manipulus units placed immediately behind the gaps of
the preceding line: rather like a chequers board. The cavalry would be
laid out to the sides.

Hastati: Principes : Triarii : Velites (spread out across the others
or as rearguard) : Equites (on the two sides).

However the standard layout was not the only strategic approach which
could be employed, a variety of shapes were employed according to the
nature of the enemy and terrain:

Cuneus (wedge) or Caput Porcinum (pig’s head): The army
was arranged in the shape of a wedge to break through enemy lines.

Globus: A circular formation, used most frequently in cases of
extremity.

Forfex: A formation like a pair of scissors, likely used if
the enemy utilized a cuneus formation in order to limit damages to their
own numbers whilst cutting into the enemy formation from the sides.

Pyrgus: A rectangular figure like a tower, with few men in
each file (row) and many files.

Serra: An approach by which the foremost manipuli would engage
for some time and retreat, allowing another set of manipuli to move
forward and engage, until they retreated to be replaced by the first
again.

Command of the Roman army can appear a little complex at first but is
actually relatively simple. Top level military command was held by the
highest public offices: the Consuls and Praetors. The senate provided
them with council and support. These public offices were elected each
year, and being elected politicians they clearly tended to lack the
acumen of seasoned military generals even though, as with all top
magisterial positions they had to have undertaken at least 10 years
worth of military service, most likely in the cavalry.

During their year of tenure these magistrates were given almost
absolute power – hence the need to introduce a check and balance system
of having two consuls often from opposite factions, two praetors (later
4), 6 military tribunes etc. The power these men held were symbolized by
bundles of rods known as fasces: a symbol of power which had been
inherited from the Etruscans in the very early days when Rome was a
kingdom.

When waging battle the lightly armoured Velites would be the ones to
open up battle, create some noise and chaos with their slings and would
eventually be ordered to fall back through the spaces left between the
manipuli of the Hastati and Principes to take their place in rear-guard
or amongst the manipuli of heavily armed infantry. At this point the two
rows of hastate and principes would spread out to create solid rows
rather than the chequered "quincunx" formation.

A quote from Virgil renders the idea (Georg. ii, 279):

"Ut saepe ingenti bello cum longa cohortes,

Explicuit Legio, & campo stetit agmen aperto,

Directaeque acies, ac late fluctuat omnis

Aere renidenti tellus, necdum horrida miscent…."

"Sed quia non aliter vires dabit omnibus aequas

Terra, neque in vacuum poterunt se extendere rami"

The two armies would then close up until they engaged in hand to hand
combat: we can imagine that as they advanced the legionaries from the
Hastati would wait until the last minute and throw their Pilums in order
to disable the enemy’s shields and indeed kill as many as possible
before having to draw their swords. Some accounts suggest that a pilum
had extremely powerful penetration capability and would often go through
enemy shields and kill the enemy soldier. The Roman soldiers would then
close in further, drawing the gladius with their right hand; the enemy
strikes with a cutting sword blow, the large Roman shields would fend
the blow with their metal edge, followed by a sharp thrust of the
shield’s central "umbo" into the enemy’s face and a stab into the
enemy’s body with the short gladius….

The cavalry would be coming in round the sides, clearly the effect of
cavalry work is far more effective when the horse can see space to
charge into rather than a compact unit of charging/shouting enemy. Thus
the difficulty of cavalry being that of keeping a cold blooded approach
and frighten the enemy formation into breaking apart to enable the horse
to wedge in, trample and the soldier to swing on those below him.